The main aims of the proposed work are to identify and characterize the effects of seasonal variations in day length on brain development, with a view toward elaborating how and why brains of animals born at different times of year come to differ structurally and functionally. The model species to be studied is the meadow vole. Specific projects include: 1) determining the postnatal intervals during which photoperiod influences brain growth, 2) assessing the influence of seasonal variation in day length on brain myelination, 3) determining the effect of photoperiod on gliogenesis and neurogenesis of developing voles, 4) assessing the effects of photoperiod and of spontaneous recrudescence of the reproductive system on brain growth of overwintering voles, 5) determining the role of testicular androgens in brain growth and in the genesis of sex differences in brain development, 6) elaborating the role of the pineal gland in photoperiod-induced brain development. Parameters to be measured or manipulated include brain mass, total brain DNA content, myelin content of brain, plasma testosterone levels, duration of day length. The proposed work may lead to new concepts of central nervous system structural modifiability by environmental factors and could ultimately result in photo- therapies that affect brain development.